Study On The Forming Process And Post-treatment Of Dental Co-cr Alloys Fabricated By Selective Laser Melting

Selective laser Melting(SLM)technology has a wide application prospect in the field of dental prostheses due to its characteristics such as personalized customization,the ability to shape complex prostheses,high forming accuracy,mass production of prostheses,and excellent physical and chemical properties.However,SLM technology also has a series of problems to be solved,such as more internal pores,poor surface roughness and large residual stress,which have been impeding the clinical application of SLM technology in the field of prosthodontics.In order to solve the above problems,this paper first characterized the microstructure of SLM Co-Cr alloy and analyzed its microstructure characteristics.Then,through experiments,the molten pool morphology,porosity,surface roughness and Vickers hardness of SLM Co Cr alloy under different forming processes(laser power P and scanning speed V)were obtained.The relationship between laser line energy density E and the microstructure and properties of the formed parts was obtained,and the printing process parameters were optimized.Then,the effects of two heat-treatment processes(hardening heat-treatment and softening heat-treatment)on the microstructure and mechanical properties of SLM Co-Cr alloy were studied for dental prostheses with different expected uses,and the effects of porcelain firing cycling(PFC)on the microstructure and mechanical properties of SLM Co-Cr alloy were also studied.The main research conclusions are as follows(1)SLM Co-Cr alloy is composed of a large number of "fish-like" clusters,which is mainly caused by the Gaussian distribution of laser energy.The inner part of the molten pool is composed of columnar sub-grains growing perpendicular to the boundary of the molten pool.This unique morphology is mainly due to the fact that the crystal nuclei are easier to form on the boundary of the molten pool,and the crystal nuclei are easier to grow in the direction of the maximum heat flow.SLM Co-Cr alloy phase is composed of single-phase supersaturated ?-Co solid solution(FCC structure),which is caused by the high temperature of molten pool and high solidification speed of SLM forming.(2)According to the influence law of laser power P and scanning speed V on the pool morphology and porosity,line energy density E was introduced to comprehensively summarize the relationship between the two process parameters P and V and the pool morphology and the performance of the formed parts.With the increase of linear energy density E,the porosity of decreases and Vickers hardness increases.However,when the laser line energy density E>0.15 J/mm,the decreasing trend of porosity and the increasing trend of Vickers hardness gradually slowed down,mainly due to the decrease of the defect initiation zone in the overlap zone of the molten pool.The surface roughness decreases first and then increases with the increase of line energy density E.When the linear energy density E<0.15 J/mm,the surface roughness of the sample is higher due to the "concave valley" phenomenon.When the linear energy density E increases to 0.15?0.2 J/mm,the surface of the sample gradually becomes smooth.When the linear energy density E continues to increase to more than 0.2 J/mm,the surface roughness of the sample increases due to the phenomenon of "convex peak".The optimized forming process parameters(linear energy density E=0.173 J/mm,laser power P=190 W,scanning speed V=1100 mm/s)were obtained by experiments.(3)Hardening and heat treatment can improve the strength and hardness of the As-SLM Co-Cr alloy,but it will reduce the plasticity of the material.This is mainly due to the isothermal FCC?HCP phase transformation and the precipitation of Laves phase in the SLM Co-Cr alloy during the hardening treatment.The softening treatment can improve the elongation of the SLM Co-Cr alloy,and also reduce the strength and hardness of the material.This is mainly due to the occurrence of recrystallization and twin boundary during the softening heat treatment.(4)The strength and hardness of the As-SLM sample were significantly increased by porcelain firing cycling treatment,while the elongation was significantly decreased.The main reason is that the crystal defects promote the appearance of the long Laves phase and the striped ?-Co phase on the sub-grain boundary,which deteriorates the toughness of the material.PFC treatment can increase the hardness and decrease the elongation of the softened samples.The change of mechanical properties is mainly due to the appearance of the Laves phase dispersed in the grain and the growth of the elongated strip Laves phase at grain boundary.PFC treatment can reduce the yield strength and tensile strength of hardened samples,and increase the elongation.It is mainly due to the HCP?FCC phase transition during PFC.